bluetooth: controller: update unittests for improved ZTEST framework

Update the unittests for feature exchange to use the new ZTEST API
Here we only update the feature exchange procedure. The remaining
unittests will be done in a next PR

Mocks, helper routines and the Unit Under Test are splitted out as
a library instead of adding them to the file list

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>